Split-rail DC from DC laptop power supply? - diyAudio
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Old 29th August 2014, 04:00 PM   #1
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Question Split-rail DC from DC laptop power supply?

Hi all,

I need a power supply for about 6 op amps. I've searched, but most online resources have a center tapped AC, then rectifier, then a 78xx/79xx pair to create +Vcc and -Vcc with virtual ground. All in all it's been a great learning experience, but I keep banging my head against a few conceptual issues.

I have a nice spare laptop power supply that gives me clean +18 VDC. Douglas Self's book gives the attached solution for a split rail power supply off an AC wall wart - could I substitute my handy dandy laptop power brick where it says "cable" and forgo the diodes in Self's schematic?

Or is there an easier solution to get +/- 15VDC off a 18VDC laptop power supply? Using a voltage divider would give me +9/0/-9, which is not enough.

Then again, because voltage is relative, why wouldn't I just be able to put 18V and 0V DC straight to the op amps? If I understand correctly, the op amp won't care if it's [-9 and +9], [0 and 18] or [100000 and 100018] for that matter. Right?

Thanks a lot
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Old 29th August 2014, 04:47 PM   #2
AndrewT is offline AndrewT  Scotland
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You have choices.
a.)
use the single polarity SMPS and create a "virtual ground" using an active circuit that has low impedance. This creates an effective +ve, zero volts and -ve supply for the opamp circuits

b.)
use the single polarity SMPS and convert the opamp circuit to use a single polarity supply.

BUT !!!!!
the ripple coming from the SMPS will be horrendous. Battery charging with a noisy SMPS does not care about the noise. The opamps working with analogue audio signals will care a lot.
You will need to use an RCRC filter between the DC output of the SMPS and the regulators. That combination should get the noise low enough that the opamps behave and process your audio signals.

If your opamps can't work with ~12Vdc (or +-6Vdc), then your SMPS is not going to work.

c.)
buy a transformer with dual secondary windings, the correct voltage and adequate VA.
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Last edited by AndrewT; 29th August 2014 at 04:50 PM.
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Old 29th August 2014, 05:43 PM   #3
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That design from Self is great. Simple, with a real ground return but relies on the input being AC. The output of your supply is DC and if you hook it up the way you suggest you'll get good voltage (not saying clean) on your positive supply, but nothing on the negative. Like Andrew said, you'll need a virtual ground to use a single ended DC supply. The designer or the O2 headphone amplifier had a great page on virtual grounds, I recommend reading it to decide if that's the way you'd like to go: NwAvGuy: Virtual Grounds & 3 Channel Amps
Otherwise, to use the Self power supply you'll need to get the transformer he suggests. It would be much cleaner since filtering switching noise can be difficult.
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Old 29th August 2014, 06:25 PM   #4
JMFahey is offline JMFahey  Argentina
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Short answer is no. Period.

If you want to use Self´s circuit you´ll need the transformer he specifies, but given that, just get the proper center tapped transformer and build a conventional supply.

Pity your circuit will not work with +/-9V ... which in fact will be even less, because you need to clean/filter them.
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Old 29th August 2014, 06:51 PM   #5
JMFahey is offline JMFahey  Argentina
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That said, you can use one of the popular charge pump ICs which provide an easy and cheap solution to getteing negative rails.

The classic TC7662 straight converts +15V to -15V .
http://ww1.microchip.com/downloads/e...Doc/21469a.pdf

If you want to get sophisticated, the LTC3260 does the same, plus gives you regulated and very clean +12V and -12V
http://www.linear.com/docs/41410

Armed with the chip numbers, search the web, somebody must have posted a practical project including some PCB ... or design your own
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Old 30th August 2014, 02:45 AM   #6
dangus is offline dangus  Canada
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Get an AC wall wart and use a half wave rectifier.

Use the MC34063 from a car cellphone charger to generate a negative supply. See the Motorola app note for schematics and board layout.
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Old 30th August 2014, 05:06 AM   #7
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Why cant he just use 0-18v as it is? Personally i would build the cirucit above with linear regulators and pickup a transformer cheap on eBay.
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Old 30th August 2014, 07:50 AM   #8
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Quote:
Originally Posted by mcandmar View Post
Why cant he just use 0-18v as it is? Personally i would build the cirucit above with linear regulators and pickup a transformer cheap on eBay.
If I understand all answers correctly (well, I hope so anyway!) then 0-18V would give me insufficient headroom (same as using +9V and -9V) and the output will be terribly noisy since the DC laptop brick I have it probably poorly regulated as the laptop brick is designed to dump raw power into a battery.

Thanks everyone for the suggestions. The single chip solutions like TC7662 will probably not give me enough power for at least 6 opamps, and I was hoping I could do without a transformer (weight is an issue), but it does seem like the cleanest/most straightforward option at the moment. Although the active virtual ground looks good, too, be it a little more complicated conceptually (thus, prone to dumb errors from my part, haha!)
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Old 30th August 2014, 08:14 AM   #9
Mooly is offline Mooly  United Kingdom
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You could use a 12 to -/+ 15 volts dc convertor. Simply regulate the laptop supply down to 12 with a 7812 and use that 12 volt feed for one of these. They come in all varieties and power outputs. Choose your opamps carefully and a 2 watt convertor may be sufficient.

This is just an example.

IH1215S - XP POWER - CONVERTER, DC/DC, 2W, +/-15V | Farnell UK
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Old 30th August 2014, 10:12 PM   #10
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Quote:
Originally Posted by Mooly View Post
You could use a 12 to -/+ 15 volts dc convertor. Simply regulate the laptop supply down to 12 with a 7812 and use that 12 volt feed for one of these. They come in all varieties and power outputs. Choose your opamps carefully and a 2 watt convertor may be sufficient.

This is just an example.

IH1215S - XP POWER - CONVERTER, DC/DC, 2W, +/-15V | Farnell UK
Thanks again! I should be calling the project "The Mooly" by now, haha.

Your IC solution is exactly what I was looking for (although I had searched I wasn't sure where to start and couldn't see the forest for the trees). I guess a 3 watt should be plenty for 6 tl072s.
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